I got the same problem with the CSharp examples in 2.4.8 "right out of the box" with VS2005. Here's my solution.
In nunit.exe.config, the following block is commented-out by default. Put it back in.
<startup>
<supportedRuntime version="v2.0.50727" />
<supportedRuntime version="v2.0.50215" />
<supportedRuntime version="v2.0.40607" />
<supportedRuntime version="v1.1.4322" />
<supportedRuntime version="v1.0.3705" />
<requiredRuntime version="v1.0.3705" />
</startup>
It sounds like you did that but changed the requiredRuntime from the default of v1.0.3705 to v2.0.50727
After that, I opened the CSharp project in VS2005 and converted it. The nunit.framework references all had the icon that shows they are wrong. So I deleted the nunit.framework reference from all 4 projects:
cs-failures
cs-money
cs-money-port
cs-syntax
Then I added new references to C:\Program Files\NUnit 2.4.8\bin\nunit.framework.dll to replace them.
After that the project builds fine.